I think that a tool that would report these info is necessarily GPU dependent.
Well, I mean that reporting the memory usage, I mean "onboard memory usage" is certainly not the fact of a standard system call.
With my nvidia proprietary driver, I use nvidia-smi

For what is the GPU usage definied by nvidia as "Percent of time over the past second during which one or more kernels was executing on
the GPU" , you'll have noticed that nvidia themselves cannot compute it... (N/A)...
OK... my GeForce is said not supported by nvidia-smi.

As for which process (I beleive you mean which linux thread) is using the GPU, I don't know what you could expect apart from the irq/whatever handler._________________